I just realized the fuel cap of my bike is rusted, I had a tough time opening it and had to come back from the gas station to open it.

the cap completely comes out and is a cylinder roughly 2 inches in dia and 2 inches in height. the part that comes in contact with the tank shows some rust. What can I put on it to prevent rusting. I am sure it will come in contact with the fuel and I don't want anything to pass in the fuel system, not rust for sure.